The invention relates generally to the field of photography, and in particular to electronic flash units for use in cameras. More specifically, the invention relates to an electronic flash unit with alternative (dual) capacitor switching to reduce flash ready time.
Electronic flash, as used in photography, is produced by an instantaneous electric discharge between two (anode and cathode) electrodes in a gas-filled glass tube. In practice, the electrical energy for the discharge is stored in a capacitor. The main components of an electronic flash unit are therefore the power supply, the capacitor, the triggering circuit, and the flash tube itself usually with a reflector.
At present, electronic flash units are built into a broad spectrum of cameras, from disposable one-time-use cameras to sophisticated single-lens reflex cameras. Typically, they use a high-frequency oscillator to convert the dc voltage from a battery to an ac voltage that""s stepped up in a transformer. This high voltage is rectified back to dc and stored in a flash (main) capacitor connected to a flash tube filled with xenon and small amounts of other gases. A monitoring circuit ignites a flash ready light when the stored voltage in the flash capacitor approaches a peak capacity. The stored voltage cannot discharge through the flash tube until the xenon becomes ionized. This happens when the camera""s shutter-flash synchronization switch closes, dumping the charge from a small trigger capacitor into an ignition coil connected to the flash tube, which produces a brief burst of voltage applied to the glass wall of the flash tube via a third (triggering) electrode. The xenon is then ionized to provide a conductive path for the stored voltage from the flash capacitor.
Since the electronic flash unit usually has only one flash capacitor, flash ready time is limited by the time it takes to recharge the flash capacitor to its storage capacity following each flash illumination. Prior art U.S. Pat. No. 5,640,624 issued Jun. 17, 1997 discloses a high-speed flash charging system that accelerates flash charging time. The flash charging system includes an energy charging section with three capacitors and a trigger section with two capacitors for energizing the flash tube. The three capacitors are successively discharged as needed to charge the two capacitors.
An electronic flash unit comprising:
a flash tube;
first and second energy-storing flash capacitors, each one having an identical storage capacity which is sufficient to energize the flash tube for the same amount of flash illumination when either one of the first and second capacitors discharges its stored energy through the flash tube; and
a controller alternatively connecting the first and second capacitors to the flash tube to permit each one of the first and second capacitors to individually discharge its stored energy through the flash tube.
Preferably, a flash charging circuit charges each one of the first and second capacitors to their identical storage capacity, and the controller connects the flash charging circuit to the first capacitor when the first capacitor has discharged its stored energy through the flash tube and connects the flash charging circuit to the second capacitor when the second capacitor has discharged its stored energy through the flash tube.